Turbulence conditioner for transit time ultrasonic flow meters and method

ABSTRACT

An apparatus for determining fluid flow in a pipe includes an ultrasonic flowmeter which communicates with the interior of the pipe through at least one pair of apertures, where each aperture of the one pair of apertures has an effective diameter. The apparatus includes a turbulence conditioner disposed in the pipe having openings where the pitch between openings is a function of the effective diameter of the aperture. A turbulence conditioner for a pipe has openings and a pitch. The conditioner has walls between the openings whose thickness is a function of the pitch. The conditioner has a length which is a function of the pitch. A method for determining fluid flow in a pipe includes the steps of flowing fluid through a turbulence conditioner disposed in the pipe having openings where the pitch between openings, is made a function of an effective diameter of an aperture of an ultrasonic flowmeter which communicates with the interior of the pipe through the aperture. There is the step of measuring the flow with the meter. A method for producing a turbulence conditioner.

FIELD OF THE INVENTION

The present invention is related to the altering of the structure ofturbulence in a pipe such that the turbulent variations of the fluidvelocity, as measured by a transit time ultrasonic flow meter downstreamof a turbulence conditioner, are much reduced. (As used herein,references to the “present invention” or “invention” relate to exemplaryembodiments and not necessarily to every embodiment encompassed by theappended claims.) The reduction in turbulent velocity variationsfacilitates the “proving” of the ultrasonic meter—confirming itscalibration against a standard—making it possible to confirm thiscalibration with significantly fewer runs of the prover than is possiblewith the same ultrasonic meter and the same prover operating without theturbulence conditioner.

BACKGROUND OF THE INVENTION

This section is intended to introduce the reader to various aspects ofthe art that may be related to various aspects of the present invention.The following discussion is intended to provide information tofacilitate a better understanding of the present invention. Accordingly,it should be understood that statements in the following discussion areto be read in this light, and not as admissions of prior art.

Transit time ultrasonic flowmeters have exhibited excellentrepeatability and absolute accuracy in many flow measurementapplications. However, characteristics inherent in the nature of theirmeasurements present difficulties when these meters are applied tocustody transfer measurements of petroleum products. A custody transfertakes place when ownership of a batch of a particular product changes.On a small scale, such a transfer takes place at the pump in a gasstation, between the owner of the gas station and his customer.

It is industry practice in custody transfer measurements to “prove” themeter; that is, to establish its calibration accurately, by independentmeans. Provers are usually devices of fixed and precisely establishedvolume. The time required to deliver the volume of product defined bythe prover is accurately defined by the transit of a ball or piston,pushed by the product, from one end of the prover to the other. Highspeed diverter valves initiate the prover run and bypass the prover whenthe ball reaches the end of its travel. Position switches at thebeginning and end of the prover synchronize the proving operation withthe operation of the custody transfer meter—the meter to be used tomeasure the amount of product delivered to a specific customer. Thevolumetric output measured by the custody transfer meter (in traditionalpractice, a turbine or positive displacement meter) during the provingrun is compared to the volume of the prover and a meter factor (i.e., acalibration correction) is established.

It is also industry practice to perform a set of several proverruns—five is typical—to establish the “repeatability” of the meterfactor of the custody transfer meter. Repeatability in the petroleumindustry is usually defined as follows: the difference between the highand low meter factors from a set of prover runs, divided by the lowmeter factor from that set. The repeatability (or in statisticalterminology, “range”) of a set of proving runs is a measure of theuncertainty of the meter factor as determined by the average of theresults of that set of runs. For example, a repeatability of 0.05% in 5runs of the prover indicates that the true meter factor for the custodytransfer meter lies within a ±0.027% band of the mean meter factor fromthat run set, with 95% confidence. A meter factor of this accuracy isthe accepted standard for custody transfer measurement.

Unlike turbine and positive displacement flow meters, a transit timeultrasonic flow meter does not measure volumetric flow ratecontinuously, but instead infers it from multiple samples of fluidvelocity. Specifically, the volumetric flow rate is determined fromperiodic measurements of the axial fluid velocity as projected onto oneor more acoustic paths—paths along which the transit times of pulses ofultrasound are measured. The path velocity measurements are combinedaccording to rules appropriate to their number and location in the pipe.Many meters employ parallel chordal paths arranged in accordance with aspecific method of numerical integration.

The period over which an ultrasonic transit time meter collects a singleset of velocity measurements (one velocity measurement or more,depending on the number of paths) is determined by the path transittimes, the number of paths, and the data processing capabilities of themeter itself. For liquid meters, the flow samples will typically becollected over periods ranging from 5 to 100 milliseconds, resulting insample frequencies between 10 Hz and 200 Hz. These figures may differfrom one ultrasonic meter design to another.

An ultrasonic flow measurement is thus a sample data system on twocounts:

-   -   (1) It does not measure the velocity everywhere across the pipe        cross section but only along the acoustic paths, and    -   (2) It does not measure velocity continuously, but instead takes        a series of “snapshots” of the velocity from which it determines        an average.

Because of these properties, a transit time ultrasonic meter responds toflow phenomena like turbulence differently than other meters commonlyused for custody transfer in the petroleum industry. More specifically,the individual flow measurements of transit time ultrasonic meters willbe affected by the small scale random (i.e., turbulent) variations inlocal fluid velocity. These variations are both temporal and spatial,and an ultrasonic instrument must make multiple measurements todetermine the true average flow rate—to reduce the random errorcontributions due to turbulence to acceptable levels. Turbine meters andpositive displacement meters, on the other hand, respond to the flowfield in the pipe as a whole; integration of the fluid velocity in spaceand time is inherent in the nature of their responses. On the other sideof the ledger, transit time ultrasonic meters are not encumbered byphysical limitations like bypass leakage and friction, and may thereforeprovide measurement capability over a wider range of velocity andviscosity conditions.

For custody transfer, flow meters are designed to produce pulses perunit volume of fluid that passes through them (for example, 1000pulses/barrel). The meter factor MF is given by:MF=V/NP

Here

-   -   V is the volume of the standard—the prover—between the two        position switches embedded in its walls. When a proving run is        initiated the flowing fluid is diverted through the prover and        pushes a ball or piston past the upstream switch, initiating the        run, which is terminated when the ball or piston reaches the        downstream switch    -   NP is the number of pulses produced by the meter during the        period which begins when the upstream switch is actuated (time        T1) and ends when the downstream switch is actuated (time T2).

Ultrasonic meters determine a flow rate Q in volume units per secondfrom individual measurements of fluid velocity along one or moreacoustic paths. They therefore must generate pulses by means of afrequency converter that produces pulses at a rate k exactlyproportional to the volumetric flow rate. Thus the number of pulses NPis given by:NP=kQ(T2−T1)

If the uncertainties in the volume of the standard, the frequencyconverter k, and the actuations of the upstream and downstream switchesare ignored (these terms are generally smaller by an order of magnitudethan the uncertainties associated with the flow instrument calibration.In more detailed analyses they are not ignored), the per unituncertainty in meter factor for a 95% confidence level is given by:dMF/MF=2dQ(N)/Q=2σ_(mean)(N)

-   -   Where dQ(N) is one standard deviation of the mean of the N flow        samples collected during the prove, or σ_(mean)(N).

One standard deviation of the mean, σ_(mean)(N), of N representativeflow samples taken during a proving run is given by:σ_(mean)(N)=S/(N)^(1/2)

Here, S is the standard deviation of the population of flow samples—thequantitative characterization of the random variability, produced by theturbulence, in the individual flow measurements of the ultrasonic meter,from one flow sample to the next.

An examination of the above equation reveals the variables that must becontrolled to achieve satisfactory proving performance in ultrasonicmeters: the turbulence intensity as it affects the standard deviation ofthe flow samples, S, in combination with the number of samples, N,accumulated during each proving run. These parameters must be such thatσ_(mean)(N) is small enough to ensure that the range of measured meterfactors does not exceed the requirement. Calculations indicate that, ifσ_(mean)(N) can be made small, meters will prove successfully more than99% of the time.

Meeting these requirements is not straightforward. With a typical lineprover operating at nominal flow rate, the duration of a single provingrun is about 20 seconds, more or less. If a sample frequency of 50 Hz isassumed, the number of samples that will be collected during a provingrun is 20×50=1000. As noted in the previously referenced patent, therandom variations due to turbulence in the flow measurements of a fourpath chordal ultrasonic can be in the 1.75% range (one standarddeviation or S) though upstream piping can lead to variations as low as1.2% or as high as 3%. Substituting the 1.2% figure, 20 second provingruns will produce a σ_(mean)(N) of about 0.04%. With the this value ofσ_(mean)(N), the probability of obtaining a set of 5 proving runs withina 0.05% range is less than 40%, a figure essentially consistent withactual proving experience. Experience also confirms what calculationsshow: Higher turbulence will produce still smaller probabilities ofsuccess.

This, then, is the problem. Turbulence, such as normally encountered inpetroleum product pipelines, adversely affects the repeatability of themeter factors for transit time ultrasonic flowmeters, as measured inshort duration prover runs. Unless something is done to alter thecharacter of the turbulence, it appears that ultrasonic flowmeter meterfactors measured with conventional provers will not achieverepeatability figures meeting petroleum industry expectations.

U.S. Pat. No. 6,647,806 is based on the hypothesis put forward byDryden. (Hugh L. Dryden and G. B. Schubauer, The Use of Damping Screensfor the Reduction of Wind Tunnel Turbulence, Journal of AeronauticalScience, April 1947.) He tied the reduction in turbulence produced by aseries of one or more fine mesh screens in cascade to the production ofeddies of very small diameters whose energy was dissipated as heat in asettling chamber downstream of the screen(s). Because screens arestructurally impractical for resisting the hydraulic forces produced byliquid flow, the means proposed by the patent endeavored to produce thesame effect with relatively small holes in plates. It will be seen inthe data of Table 1 of that patent, reproduced below, that theimprovements achieved were small. The largest reductions in turbulentvariations cited in the prior patent were produced by reducers, eitheralone, or in combination with plates having small holes.

TABLE 1 Reproduced from U.S. Pat. No. 6,647,806 B1 Standard Deviation ofOne Flow Turbulence conditioner Configuration Sample Straight pipe withno diffuser mechanism 1.2% to 1.75%* Large hole perforated plate 1.61%Small hole perforated plate 0.93% Reducer immediately upstream 0.63%Reducer/large hole perforated plate 0.64% Reducer/small hole perforatedplate 0.59% *The lower figure was not included in the referenced patent,but reflects multiple measurements made subsequent to the filing of thatpatent. Standard deviations higher than 1.75% can be found 5 to 10diameters downstream of hydraulic disturbances such as bends, compoundbends, and header exits.

The method for reducing the effects of turbulence employed by theturbulence conditioners of this invention does not rely on theelimination of turbulence through the dissipation of very small eddies.Rather, the reduction in the random deviations of flow samples isbrought about reducing the eddy sizes such that they are effectivelyaveraged within the acoustic beams of the ultrasonic meter.

The reduction in eddy sizes produced by the turbulence conditioners ofthis invention also leads to an increase in the frequencies of therandom variations in fluid velocity produced by the turbulence. Thefrequency increases also lead to improved proving performance, by makinga limited sample of N velocity measurements collected during a provingrun more representative of the entire population of velocity variations.

SUMMARY OF THE INVENTION

The present invention pertains to an apparatus for determining fluidflow in a pipe. The apparatus comprises an ultrasonic flowmeter whichcommunicates with the interior of the pipe through at least one pair ofapertures, where each aperture of the one pair of apertures has aneffective diameter. The apparatus comprises a turbulence conditionerdisposed in the pipe having openings where the pitch between openings isa function of the effective diameter of the apertures.

The present invention pertains to a turbulence conditioner for a pipe.The conditioner has openings and a pitch. The conditioner has wallsbetween the openings whose thickness is a function of the pitch. Theconditioner has a length which is a function of the pitch.

The present invention pertains to a method for determining fluid flow ina pipe. The method comprises the steps of flowing fluid through aturbulence conditioner disposed in the pipe having openings where thepitch between openings is made a function of an effective diameter of anaperture of an ultrasonic flowmeter which communicates with the interiorof the pipe through the aperture. There is the step of measuring theflow with the meter.

The present invention pertains to a method for producing a turbulenceconditioner for use with an ultrasonic flow meter in a pipe. The methodcomprises the steps of identifying an effective diameter of an aperturein the pipe through which the flowmeter communicates with the interiorof the pipe. There is the step of determining a pitch between holes inthe conditioner as a function of the effective diameter.

The present invention pertains to an apparatus for determining fluidflow in a pipe, comprising an ultrasonic transit time flowmeter and aturbulence-altering turbulence conditioner, both placed in the pipe. Thealteration of the turbulence by the apparatus is such that a metercalibration meeting very narrow accuracy requirements can be determinedin a few runs of a prover (a volumetric standard), a capability nototherwise achievable. Several turbulence conditioner configurations thatwill produce the necessary alteration to the turbulence are described. Asecond arrangement of the apparatus is also described. It draws on aconfiguration employing a reducing nozzle downstream of the turbulenceconditioner but upstream of the meter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a honeycomb conditioner viewed from upstream with transducerapertures of chordal meter shown in phantom downstream.

FIG. 1B is sectional view of the honeycomb conditioner (the chordalmeter is not shown but couples to lower flange).

FIG. 2 is a perspective drawing of the honeycomb turbulence conditioneralso as prescribed herein, with the chordal ultrasonic meter downstream.

FIG. 3A is a perspective drawing of an “egg-crate” turbulenceconditioner also as prescribed herein, with the chordal ultrasonic meterdownstream.

FIG. 3B is a sectional view of the “egg-crate” turbulence conditioner.

FIG. 3C is a side sectional view of the “egg-crate” turbulenceconditioner.

FIG. 4A is a perspective drawing of a “mini-tube” turbulence conditioneralso as prescribed herein, with a reducing nozzle and chordal meterdownstream (and an expanding nozzle downstream of the meter, to maintainupstream and downstream pipe diameter).

FIG. 4B is a sectional view of the “mini-tube” turbulence conditioner.

FIG. 4C is a side sectional view of the “egg-crate” turbulenceconditioner.

FIG. 5 is a sketch showing turbulent eddies intersecting an ultrasonicbeam, (a) without a turbulence conditioner and (b) with the turbulenceconditioners of this invention.

FIG. 6 is a plot of experimental results obtained with the honeycombturbulence conditioner and flowmeter as described herein, showing themagnitude of the flow variations, as measured by one standard deviationof the flow samples, as a function of flow rate. The magnitude ofvariations that would typically be obtained without the turbulenceconditioner is also shown.

FIG. 7 is a plot of experimental results, showing the average range of aset of five meter factor measurements obtained with the honeycombturbulence conditioner and ultrasonic flowmeter as described herein, asa function of the calculated standard deviation of the mean flow duringa proving run, for proving volumes typical of the flow rates presentduring the measurements.

FIG. 8 is a bar chart of experimental results showing the percentage ofsuccessful sets of 5 proving runs each, as a function of the calculatedstandard deviation of the mean flow during the proving runs.

FIG. 9 is a cross-section of a conditioner with different sized holes.

FIGS. 10 and 11 show a conditioner where the holes are not of equaldimensions.

FIG. 12 is a cross-section of a conditioner with holes having 2 sides.

DETAILED DESCRIPTION

Referring now to the drawings wherein like reference numerals refer tosimilar or identical parts throughout the several views, and morespecifically to FIGS. 1A, 1B, 2, 3A, 3B, 3C, 4A, 4B and 4C thereof,there is shown an exemplary apparatus 10 for determining fluid flow in apipe 12. The apparatus 10 comprises an ultrasonic flowmeter 14 whichcommunicates with the interior of the pipe 12 through at least one pairof apertures 18, where each aperture 18 of the one pair of apertures 18has an effective diameter. The apparatus 10 comprises a turbulenceconditioner 16 disposed in the pipe 12 having openings 22 where thepitch between openings 22 is a function of the effective diameter of theaperture 18.

The conditioner 16 has walls 20 between the openings 22 whose thicknesscan be a function of the pitch. The conditioner 16 has a length whichcan be a function of the pitch. The meter 14 can be disposed up to 3internal pipe 12 diameters downstream of the conditioner 16. The pitchcan be less than the effective diameter of the aperture 18. The walls 20can have a thickness between ¼ and 1/10 of the pitch. The length can be5 to 20 times the pitch. The pitch can be a function of the meter's 14diameter. The pitch can be a function of the meter's 14 maximumvelocity. The pitch can be a function of the meter's 14 sampling rate.

The holes can have a cross-section that is two or more sided. FIG. 12shows holes that are two sided. Alternatively, the holes can have acircular cross-section, as shown in FIGS. 4 a and 4 b. A filter 26 canbe disposed in the pipe 12 upstream from the conditioner 16. A nozzle 24can be disposed in the pipe 12 upstream of the flowmeter 14 anddownstream of the conditioner 16. The meter 14 and the conditioner 16can satisfy proving requirements for maximum and minimum flow velocitiesfor which the meter is designed. The conditioner 16 and the meter 14 cansatisfy a proving requirement of +/−0.027% uncertainty in five provingruns.

The present invention pertains to a turbulence conditioner 16 for a pipe12, as shown in FIGS. 1A, 1B, 2, 3A, 3B, 3C, 4A, 4B and 4C. Theconditioner 16 has openings 22 and a pitch. The conditioner 16 has walls20 between the openings 22 whose thickness is a function of the pitch.The conditioner 16 has a length which is a function of the pitch.

The present invention pertains to a method for determining fluid flow ina pipe 12. The method comprises the steps of flowing fluid through aturbulence conditioner 16 disposed in the pipe 12 having openings 22where the pitch between openings 22, is made a function of an effectivediameter of an aperture 18 of an ultrasonic flowmeter 14 whichcommunicates with the interior of the pipe 12 through the aperture 18.There is the step of measuring the flow with the meter 14.

The flowing fluid through a turbulence conditioner 16 step can includethe step of flowing the fluid through the turbulence conditioner 16wherein the conditioner 16 has walls 20 between the openings 22 whosethickness is made a function of the pitch. The flowing fluid through aturbulence conditioner 16 step can include the step of flowing the fluidthrough the turbulence conditioner 16 wherein the conditioner 16 has alength which is made a function of the pitch. The flowing fluid througha turbulence conditioner 16 step can include the step of flowing thefluid through the turbulence conditioner 16 disposed up to 3 internalpipe 12 diameters upstream of the conditioner 16.

The flowing fluid through a turbulence conditioner 16 step can includethe step of flowing the fluid through the turbulence conditioner 16wherein the pitch is less than the effective diameter of the aperture18. The flowing fluid through a turbulence conditioner 16 step caninclude the step of flowing the fluid through the turbulence conditioner16 wherein the walls 20 have a thickness between ¼ to 1/10 of the pitch.The flowing fluid through a turbulence conditioner 16 step can includethe step of flowing the fluid through the turbulence conditioner 16wherein the length of the conditioner 16 is 5 to 20 times the pitch.

The flowing fluid through a turbulence conditioner 16 step can includethe step of flowing the fluid through the turbulence conditioner 16wherein the holes have a cross-section that is two or more sided.Alternatively, the flowing fluid through a turbulence conditioner 16step includes the step of flowing the fluid through the turbulenceconditioner 16 wherein the holes have a circular cross-section.

The flowing fluid through a turbulence conditioner 16 step can includethe step of flowing the fluid through a filter 26 disposed in the pipe12 upstream from the conditioner 16. The flowing fluid through aturbulence conditioner 16 step can include the step of flowing the fluidthrough a nozzle 24 disposed in the pipe 12 upstream of the flowmeter 14and downstream of the conditioner 16. The measuring step can include thestep of measuring the flow with the meter 14 which meets a provingrequirement. The measuring step can include the step of measuring theflow with the meter 14 which meets the proving requirement of +/−0.027%uncertainty in five proving runs.

The present invention pertains to a method for producing a turbulenceconditioner 16 for use with an ultrasonic flowmeter 14 in a pipe 12. Themethod comprises the steps of identifying an effective diameter of anaperture in the pipe 12 through which the flowmeter 14 communicates withthe interior of the pipe 12. There is the step of determining a pitchbetween holes in the conditioner 16 as a function of the effectivediameter.

There can be the step of determining a thickness of walls between theopenings as a function of the pitch. There can be the step ofdetermining a length of the conditioner 16 as a function of the pitch.There can be the step of building the conditioner 16 having the pitch,wall thickness and length determined in the determining steps. Thebuilding step could include fabricating the center portion 35 having theholes 22, cutting it to length and diameter to meet the designparameters determined for the pitch, wall thickness and length, andattaching a flange 37 to the center portion 35 so it can be seated inthe pipe 12.

The present invention pertains to an apparatus 10 for determining fluidflow in a pipe 12, comprising a turbulence-altering turbulenceconditioner 16 and an ultrasonic transit time flowmeter 14, both placedin the pipe 12. The arrangement facilitates the “proving” of theultrasonic meter 14 in a number of prover runs comparable to, or betterthan the number required by meters of competing technologies (turbinemeters and positive displacement meters)—an accomplishment not possiblewith conventional arrangements of ultrasonic meters 14. Severalalternative configurations for the turbulence conditioner 16 aredescribed. The turbulence conditioners 16 can also be applied to analternative arrangement of the enclosing piping and ultrasonic meter 14which has been described in the prior art, and in which a nozzle 24 typereducer is employed upstream of the ultrasonic meter 14 but downstreamof the turbulence conditioner 16, with proving results better thanpreviously achieved with this arrangement.

FIGS. 1A, 1B and 2 show one of the several alternative turbulenceconditioners 16 employed by this invention with a chordal ultrasonicmeter 14 downstream, as prescribed by this invention. The configuration,referred to as a “honeycomb” having six sides, has been tested todemonstrate the capabilities of this invention. Results will bedescribed in later paragraphs. The key dimensional requirements of thisexemplary embodiment of the invention are as follows:

-   -   1. In the configuration shown, the pitch of the honeycomb        turbulence conditioner 16—the spacing between the centers of the        openings 22—is one-half the effective diameter of the apertures        18 of the ultrasonic meter 14 (¼ inch pitch vs. ½ inch        aperture); although generally, the pitch should be less than the        effective diameter of the aperture 18. The aperture 18 diameter        is equal to the diameter of the transducer assembly if the bores        of the pipe 12 penetrations containing the ultrasonic transducer        assemblies are equal or nearly equal to the diameters of the        transducer assemblies. In some ultrasonic meter 14 designs, the        minor diameter of the opening 22 in the internal wall 20 of the        pipe 12 through which the ultrasonic energy passes is smaller        than the diameter of the transducer assembly. In such cases,        this minor diameter of the opening 22 is the effective diameter        of the aperture 18. Substantial reductions in flow variations        have been obtained with the ¼ inch pitch to ½ inch aperture 18        ratio. However, even greater reductions may be obtained if the        pitch to aperture 18 ratio is smaller than one-half.    -   2. The thickness of the walls 20 of the honeycomb is a small        fraction of the pitch (in the order of 1/10). Calculations show        that walls 20 that are thin relative to the pitch of the        turbulence conditioner 16—in the range of ¼ to 1/10 of the        pitch—will provide adequate structural strength for the        turbulence conditioner 16. Because the walls 20 are thin, the        eddies generated in their lee are of extremely small        diameter—small enough not to impact the net statistics of the        flow velocities measured by the ultrasonic meter 14 (because the        eddies are very small relative to the diameter of the aperture        18). In this regard the honeycomb configuration and the other        turbulence conditioner 16 configurations covered in this        invention are inherently superior to the plate conditioners 16        of the previously referenced patent, where the dimensions of the        ligaments are in the same order as the holes.    -   3. The axial length of the turbulence conditioner 16 is in the        range of 5 to 20 times the pitch. This length is sufficient to        eliminate global vortices created by features of the piping        upstream of the conditioner 16, obviating the need for        additional flow conditioning. Turbulence conditioners 16        employing large diameter tubes or plates with holes are normally        provided for turbine and ultrasonic meters 14 to eliminate        global vortices. With the proposed turbulence conditioners 16        these will not be necessary. The 5:1 to 20:1 length to pitch        ratio also provides a configuration that is axially stiff,        thereby producing a structure that does not deflect        significantly in the flow stream and that is strong enough to        withstand flow forces. Calculated pressure drops of the        turbulence conditioners 16 described herein are comparable to        the pressure drops of conventional tube type flow conditioners.        The pressure drops of the turbulence conditioners 16 described        herein are less than the pressure drops of plate type turbulence        conditioners 16 such as described in the previously referenced        patent.    -   4. In FIG. 2, the ultrasonic meter 14 is located approximately        one internal pipe 12 diameter downstream of the turbulence        conditioner 16. Data discussed later show that marked        improvements in proving performance are achieved with this        configuration. A separation in the range of up to 3 internal        pipe 12 diameters will produce satisfactory performance. Larger        separation will allow eddies to agglomerate into larger eddies        and new eddies to form, reducing the effectiveness of the        acoustic beams of the ultrasonic meter 14 in averaging the        rotational velocities of the turbulent eddies. Axial distances        shorter than ½ diameter can cause local velocity profile        perturbations produced or preserved by the turbulence        conditioner 16 to be “seen” by the ultrasonic meter 14,        degrading its capability to integrate the axial velocity profile        numerically.

FIGS. 3A, 3B, 3C, 4A, 4B and 4C show alternative configurations forturbulence conditioners 16 meeting the requirements prescribed above.For each design, the axial length of the conditioner 16 providesstiffness which reduces axial deflections and stresses to very smallvalues provided the egg-crate of FIGS. 3A, 3B, and 3C and the tubebundle of FIGS. 4A, 4B, 4C can be made to act as a plate such that thetensile stresses on the downstream end can be transmitted from tube totube. For the tube bundle design of FIGS. 4A, 4B, and 4C, this isaccomplished by redundant means:

-   -   (a) A shrink fit, produced by heating the enclosing pipe 12        100° F. before inserting the bundled tubes, produces frictional        forces between tubes capable of withstanding the hydraulic        forces tending to dislodge them, and    -   (b) Spot welds between adjacent tubes near the downstream end of        the bundle, ensures that the bundle acts as a single structure.        Alternatively, the unions among tubes can be achieved by        soldering.

Spot welds would also be used to stiffen the downstream corners of theegg-crate conditioner 16 of FIGS. 3A and 3B.

Because petroleum pipelines are not free of debris (for exampleprecipitated wax or asphaltenes) and because of the relatively smallopenings 22 of the conditioners 16 prescribed in this invention, it isbeneficial to install a basket type flow filter 26 or fine mesh strainerfive or more diameters upstream of the turbulence conditioners 16prescribed herein. The aggregate pressure loss generated by the filter26 or strainer, the turbulence conditioner 16, and the ultrasonicflowmeter 14 itself is less than that of competing technologies. Aturbine meter 14 also benefits from a similar filter 26 and a flowconditioner having a similar pressure drop and itself generates apressure drop, which the ultrasonic meter 14 does not. A positivedisplacement meter 14 also generally has a filter 26 and though itgenerally has no turbulence or flow conditioner, itself generates apressure loss greater than that produced by the turbulence conditioners16 described herein.

FIGS. 5A and 5B illustrate the principles of this invention. Transittime ultrasonic meters measure the travel times of pulses of ultrasonicenergy transiting from a transmitting transducer to a receivingtransducer. In the figures, a transmitting transducer has generated apulse that is traveling diagonally across the flow stream in thedirection of flow. The pulse's transit time is given by the quotient ofthe distance between transducers and the propagation velocity of theultrasound. For a transmission in the direction shown in the figure, thepropagation velocity is the sum of the velocity of ultrasound in thefluid at rest and the fluid velocity projected onto the acoustic path.When a second transmission is generated in the opposite direction (thatis, from the downstream transducer in the figure to the upstreamtransducer) the propagation velocity is the difference between thevelocity of ultrasound in the fluid at rest and the fluid velocityprojected onto the acoustic path. Thus, knowing the path length, the twotransit time measurements yield two equations in two unknowns: theultrasound velocity in the fluid at rest and the fluid velocityprojected onto the acoustic path. But, as illustrated in FIG. 5A, thefluid velocity projected onto the acoustic path is determined not onlyby the projection of the average axial velocity (the variable that needsto be measured) but also the net projection from the multiple turbulenteddies that accelerate or retard the pulse on its way. The transmissionin the opposite direction is also affected by the turbulence, but notgenerally by the same eddies (which may have moved on betweentransmissions). For the condition illustrated in FIG. 5A, therefore,multiple measurements are needed to average out the effects of theturbulent eddies and thereby to determine the average axial fluidvelocity along the path.

When the maximum eddy diameter is made significantly smaller than thediameter of the transducer apertures that form the acoustic paths alongwhich the pulses of ultrasound travel, the disturbances to theultrasound propagation velocity produced by the tangential velocity ofan eddy tend to be canceled in each transmission. This is illustrated inFIG. 5B. Within the beam of ultrasound, the component of an eddy thathastens the ultrasound transmission is offset by the component of thesame eddy that retards it. Eddies typically move at a velocity close tothe average fluid velocity—typically in the order of 100 inches/second.The propagation velocity of the ultrasound is in the order of 50,000inches/second. Thus the displacement, during a pulse transit, of an eddywithin the 0.7 inch projected axial width of a (typical) 0.5 inchultrasonic transmission beam is negligibly small. Thus the rule is thatthe turbulence conditioner design should allow the passage of only thoseeddies which are significantly smaller than the aperture diameter andshould break up larger eddies such that only eddies whose diameter issignificantly smaller than the aperture are produced.

As noted above, in the present invention, the random deviations of flowsamples of the ultrasonic meter 14 are made small by reducing theturbulent eddy sizes such that they are effectively averaged within theacoustic beams of the meter 14. The data of FIG. 6 show the reductionproduced by a ¼ inch honeycomb turbulence conditioner 16, as shown inFIG. 1, in a 6 inch duplex ultrasonic meter 14 (the duplex meter 14consists of two essentially independent meters 14 of four paths each),having transducer apertures 18 of approximately ½ inch diameter. It willbe seen that, over a 9:1 range of flows, the turbulence conditioner 16produces standard deviations for individual flow samples ofapproximately ½% (versus a standard deviation of 1.2%, which would beexpected from the same meter 14 operating in a straight pipe 12 withoutthe turbulence conditioner 16).

The reduction in the standard deviation in the flow samples translatesinto improved proving performance. The proving performance with thehoneycomb conditioner 16 was measured by examining the range of meter 14factors in sets of 5 proving runs each. Ten sets of 5 run trials werecarried out for each of the two 4 path meters 14, at two different flowrates and with two different proving volumes—a total of thirty 5 runsets for each of the two meters 14. For twenty of these run sets aprover volume of 20 barrels was employed, 10 sets each at 3400 and 2700barrels per hour respectively. For the remaining runs a prover volume of10 barrels with a flow rate of 3400 barrels per hour was employed.Proving volumes in the 10 to 20 barrel range are typical for the twoflow rates.

Proving data collected during the tests with the turbulence conditioner16 of FIG. 1 are presented in FIG. 7. The abscissa of FIG. 7 is thestatistical characterization of the flow variability developed in apreceding section: 2×σ_(mean), twice the quotient of the standarddeviation of the individual flow samples measured during a proving runand the square root of the number of samples N taken during that run. Nis computed as the product of the ultrasonic meter 14 sample rate (about50 Hz) and the duration of the run. The ordinate of FIG. 7 is theaverage range of the proving runs. A linear trend of the data shows thatthe average range of the data from a set of 5 proving runs isapproximately equal to 2×σ_(mean).

This trend line can be used to project proving performance. If twostandard deviations of the measurements are ±0.025%, a normaldistribution shows that the odds that one run will fall outside a 0.05%band encompassing the mean are 1 in 20. The odds that any one of theremaining four runs will fall outside the opposite end of the band are 1in 40. Thus the odds of obtaining two results differing by more than0.05% are: ( 1/20)×( 1/40)+( 1/20)×( 1/40)+( 1/20)×( 1/40)+( 1/20)×(1/40)=0.005. With 2×σ_(mean.)=0.025%, sets of five proving runs shouldfall within the prescribed 0.05% range 99.5% of the time.

FIG. 8 shows the percentage of 5 run prove sets that were actuallysuccessful—that met the requirement of 0.05% range for 5 proves. Asprojected above, the figure shows that, when 2×σ_(mean) is 0.025% orless, 100% of the 5 run sets were successful. When 2×σ_(mean) is in the0.045% range, about 50% to 70% of the 5 run sets are successful. Theselatter results are also roughly in accordance with statisticalpredictions.

The performance of FIGS. 7 and 8 is markedly better than would beobtained with the same chordal flowmeter 14 without the turbulenceconditioner 16. Without the conditioner 16, 2 σ_(mean) would be in the0.06% range, at best, for the 20 barrel proves, implying a success rateof less than 50%, which is also in agreement with experience.

As has been previously noted, reducing nozzles 24, such as described inthe patent previously referenced (U.S. Pat. No. 6,647,806, incorporatedin its entirety by reference herein), can be applied with the turbulenceconditioners 16 described herein to enhance proving performance stillfurther. FIG. 4 shows a tube bundle turbulence conditioner 16 meetingthe dimensional parameters prescribed herein upstream of a reducingnozzle 24, with an ultrasonic meter 14 downstream of the nozzle 24. Anexpanding diffuser downstream of the meter 14 restores the upstream pipe12 diameter at minimal loss. In the flow nozzle 24 configuration, theultrasonic flow meter 14 is located in the cylindrical throat of thenozzle 24, which increases the mean fluid velocity seen by the meter 14as the reciprocal of the beta ratio squared. The beta ratio is thequotient of the nozzle 24 throat diameter and the upstream pipe 12diameter. The reduction in turbulence comes about because, while thenozzle 24 increases the average fluid velocity, it does notsignificantly alter the tangential velocities of the turbulent eddies.Thus the turbulence as a percentage of average fluid velocity isreduced. Tests have demonstrated the ability of nozzles 24 having a betaratio of 0.67 to reduce the standard deviation of individual flowmeasurements by about a factor of two, as might be expected since(1/0.67)²≈½. The use of a nozzle 24 downstream of the turbulenceconditioner 16 but upstream of the flowmeter 14 would reduce thestandard deviations of the flow of FIG. 6 from 0.5% to 0.25%. Thearrangement would halve the (2 σ_(mean)) data of FIGS. 7 and 8, leadingto 100% success even with the 10 barrel proving volume.

It should be noted that the pitch does not have to be fixed. FIG. 9shows larger holes in some part of the cross section of the conditioner16. FIGS. 10 and 11 show the openings 22 are not of equal dimensions,but vary.

Although the invention has been described in detail in the foregoingembodiments for the purpose of illustration, it is to be understood thatsuch detail is solely for that purpose and that variations can be madetherein by those skilled in the art without departing from the spiritand scope of the invention except as it may be described by thefollowing claims.

1. An apparatus for determining fluid flow in a pipe having openingscomprising: an ultrasonic flow meter which communicates with theinterior of the pipe through at least one pair of apertures, each ofwhich is disposed in a respective opening in the pipe, where eachaperture of the one pair of apertures has an effective diameter; and aturbulence conditioner disposed in the pipe, the conditioner havingopenings where the pitch between the conditioner openings is less thanthe effective diameter of the aperture.
 2. An apparatus as described inclaim 1 wherein the conditioner has walls between the openings whosethickness is a function of the pitch.
 3. An apparatus as described inclaim 2 wherein the conditioner has a length which is a function of thepitch.
 4. An apparatus as described in claim 3 wherein the meter isdisposed up to 3 internal pipe diameters downstream of the conditioner.5. An apparatus as described in claim 4 wherein the pitch is less thanthe effective diameter of the aperture.
 6. An apparatus as described inclaim 5 wherein the walls have a thickness between ¼ to 1/10 of thepitch.
 7. An apparatus as described in claim 6 wherein the length is 5to 20 times the pitch.
 8. An apparatus as described in claim 7 whereinthe openings of the conditioner have a cross-section that is two or moresided.
 9. An apparatus as described in claim 7 wherein the openings ofthe conditioner have a circular cross-section.
 10. An apparatus asdescribed in claim 7 including a filter disposed in the pipe upstreamfrom the conditioner.
 11. An apparatus as described in claim 10including a nozzle disposed in the pipe upstream of the flow meter anddownstream of the conditioner.
 12. An apparatus as described in claim 11wherein the pitch is a function of the meter's maximum velocity.
 13. Amethod for determining fluid flow in a pipe having openings comprisingthe steps of: flowing fluid through a turbulence conditioner disposed inthe pipe, the conditioner having openings where the pitch between theconditioner openings, is less than an effective diameter of an apertureof an ultrasonic flowmeter which communicates with the interior of thepipe through the aperture disposed in an opening in the pipe; andmeasuring the flow with the meter.
 14. A method as described in claim 13wherein the flowing fluid through a turbulence conditioner step includesthe step of flowing the fluid through the turbulence conditioner whereinthe conditioner has walls between the openings whose thickness is made afunction of the pitch.
 15. A method as described in claim 14 wherein theflowing fluid through a turbulence conditioner step includes the step offlowing the fluid through the turbulence conditioner wherein theconditioner has a length which is made a function of the pitch.
 16. Amethod as described in claim 15 wherein the flowing fluid through aturbulence conditioner step includes the step of flowing the fluidthrough the turbulence conditioner disposed up to 3 internal pipediameters upstream of the conditioner.
 17. A method as described inclaim 16 wherein the flowing fluid through a turbulence conditioner stepincludes the step of flowing the fluid through the turbulenceconditioner wherein the pitch is less than the effective diameter of theaperture.
 18. A method as described in claim 17 wherein the flowingfluid through a turbulence conditioner step includes the step of flowingthe fluid through the turbulence conditioner wherein the walls have athickness between ¼ to 1/10 of the pitch.
 19. A method as described inclaim 18 wherein the flowing fluid through a turbulence conditioner stepincludes the step of flowing the fluid through the turbulenceconditioner wherein the length is 5 to 20 times the pitch.
 20. A methodas described in claim 19 wherein the flowing fluid through a turbulenceconditioner step includes the step of flowing the fluid through theturbulence conditioner wherein the openings of the conditioner have across-section that is two or more sided.
 21. A method as described inclaim 19 wherein the flowing fluid through a turbulence conditioner stepincludes the step of flowing the fluid through the turbulenceconditioner wherein the openings of the conditioner have a circularcross-section.
 22. A method as described in claim 19 wherein the flowingfluid through a turbulence conditioner step includes the step of flowingthe fluid through a filter disposed in the pipe upstream from theconditioner.
 23. A method as described in claim 19 wherein the flowingfluid through a turbulence conditioner step includes the step of flowingthe fluid through a nozzle disposed in the pipe upstream of theflowmeter and downstream of the conditioner.
 24. A method as describedin claim 19 wherein the measuring step includes the step of measuringthe flow with the meter which meets a proving requirement.